Control circuits for shutting off power to a pumping well when the well has been pumped dry have generally relied on either a change in an average measurement, such as the average motor current, average load or average time between load changes, or in a change in the rate of change (slope) of the motor current or load.
The present invention uses the actual load on the polished rod and the actual position of the polished rod to determine if the pump is filling properly and utilizes the fact that as the pump begins to only partially fill, the load on the downstroke of the pump remains large. The invention employs a novel approach by allowing an easy method of setting a point for checking if the pump is filling properly.
Another problem is that should the rods which connect the pump with the surface break in two due to wear, the pumping unit should be shut down and an alarm set. The invention checks for this occurrence on every stroke by comparing the upstroke load against a preset load. If the load is less than the set point and the position is equal to a set point on the upstroke, indicating a lighter load due to the broken rods, the control circuit shuts the pumping unit off and sets a visual alarm to alert the problem. The present invention allows the use of an external plotter which plugs into the control circuitry and allows the operator to obtain a plot of the actual operation of the pump. The external plotter is also used to set the point at which the control circuitry checks the pump fillage and the condition of the rods.